The present invention relates to a method for producing a fibrous sheet and more particularly to the so-called internal application of latex in production of a fibrous sheet by known wet paper making technique or wet non-woven fabric making technique which comprises previously flocculating a chlorinated polymer latex into a flocculate of 100-500.mu. in particle size, adding the resultant flocculate to a fiber slurry which is separately prepared, making the slurry into a sheet and drying the resultant sheet.
The method for internal application of latex is roughly classified into the following two techniques.
(1) The so-called beater addition which comprises flocculating a latex in a fiber slurry to deposite the latex onto the surface of the fibers. PA0 (2) The method which comprises previously producing a flocculate of a latex and adding the flocculate to a fiber slurry which has been separately prepared. (In this case, said flocculate is retained in the sheet by filtering action.)
The present invention belongs to the technical field (2) and a novel method for controlling the particle size of chlorinated polymer latex such as anionic polyvinyl chloride (referred to as "PVC" hereinafter) latex, polyvinylidene chloride (referred to as "PVDC" hereinafter) latex, or combination thereof which has been difficult to attain has been found.
Hitherto, said method (1) which comprises adding a latex to a beater has been carried out in the production of papers and boards. However, according to such method, various troubles in paper making are apt to occur when the latex is added in a great amount of more than 20% by weight of fiber. Therefore, said method (2) has been mainly employed in the production of non-woven fabrics where a large amount of latex is often used.
However, since the retention of the latex in the sheet depends upon only the filtering action, it is needless to say that control of the size of the latex flocculate is very important.
As the result of various experiments by the inventors, it has been found that a flocculate of 100.mu.-500.mu. in particle size is optimum although the optimum particle size may somewhat vary depending on the thickness and shape of the fibers.
A large flocculate having a particle size of more than 500.mu. shows 100% retention, but the resultant sheet has specks and tends to adhere to the surface of drier.
In the case of a flocculate of less than 100.mu. in particle size, retention is not satisfactory and the necessary strength cannot be obtained and moreover the waste water is markedly contaminated. According to the present invention, novel conditions for flocculation of chlorinated polymer latices such as anionic PVC latex and PVDC latex which are difficultly controlled in particle size of their flocculates have been found.
The particles size of the flocculate is defined as maximum diameter which passes through the center of each flocculate when observed under a microscope.
When anionic PVC and PVDC latices are flocculated with water soluble cationic polymers or polyvalent metal salts by the conventional technique, only such flocculates having a particle size of less than 50.mu., mostly 10-20.mu. are obtained even under a very slow stirring condition. As the result of the inventors' intensive research in an attempt to obtain a stable flocculate of 100.mu.-500.mu. in particle size, it has been found that a latex finally becomes a stable flocculate through the following stages.
That is, when a flocculate is produced by adding water soluble cationic polymers or polyvalent metal salt to an anionic PVC or PVDC latex, the latex grows to a flocculate of 1 mm to 10 mm for the first several seconds by the shock of the addition, thereafter redispersion of the flocculate occurs to give the form of grape bunch and then the dispersion becomes a flocculate of about 10 to 30.mu. in particle size in such a manner that individual grapes fall from the bunch.
It has also been confirmed that the same progress of flocculation as mentioned above follows when an anionic acrylic latex is flocculated with a water soluble cationic polymer under relatively mild flocculation conditions, but what is different from the flocculation of the anionic PVC and PVDC latices is that the acrylic latex is difficulty redisperred into the flocculate finer than the state of the grape bunch and so the floccuate is stabilized in the form of coarse flocculate (200.mu. to 1 mm). Furthermore, it has also been recognized that in the case of radical flocculation with an aluminum salt, the flocculates of 1 mm to 10 mm produced due to the shock of addition gather to form masses of the flocculate.
From the above results, it has been found that the adhesion power within the flocculate of latex has a remarkable influence on the particle size of the final flocculate and hence the minimum film-forming temperature (abbreviated as "MFT" hereinafter) of the latex holds the key.